Configure email script

Hi I have recently moved several websites onto a new host / server. We are using a script that has a module allowing for the generation of custom forms but the new host has / is blocking mail from sending as they say the script is configured incorrectly. I have replaced the proper email address to myclients@[login to view URL] for demostration purposes The reason it isn't possible to use PHPMail or SMTP for this is because the form is attempting to send as something other than 'myclients@[login to view URL]'. As you can see below: Jun 22 14:40:55 smtp1 exim[15091]: 2017-06-22 14:40:55 H=[login to view URL] ([login to view URL]) [[login to view URL]] F=<test@[login to view URL]> A=dovecot_cram_md5:myclients@[login to view URL] rejected RCPT <rod@[login to view URL]>: Cannot send as test@test.com. You must send as myclients@website.co.uk. For some reason, you're script/plugin is attempting to send as the address inputted into the contact form. You'll somehow need to amend/change the script to send as the email address specified.